This invention relates to AM radio squelch circuitry. Specifically it involves the use of a phase-locked loop to extract a tone which in turn gates the receiver output. Conventional squelch circuits operate by sampling the magnitude of the automatic gain control (AGC) voltage of the receiver and sending a gating voltage to an audio amplifier, so as to prevent reception of noise in the absence of a signal. The AGC voltage required for squelch operation is adjustable with a front panel control to provide silencing of the receiver at different noise levels.
Problems with this type of squelch circuit arise when varying noise levels are encountered and weak signals must be received. Also strong noise will cause the receiver to develop an AGC voltage, sometimes many times greater than the AGC voltage developed by the desired signals. Constant readjustment of the control is required under these conditions. Weak signals are often lost entirely because they do not develop enough AGC voltage to operate the squelch circuit after the control has been adjusted for high noise levels.
When a nearby transmitter is operated on a frequency (or channel) near the frequency to which the receiver is tuned, a "bleedover" or "splatter" often results. Bleedover, often called co-channel interference, occurs when the transmitter signal is so strong that the receiver cannot reject the signal, even though the signal does not extend into the frequency band to which the receiver is tuned. This is a shortcoming of many inexpensive receivers. Splatter, often called adjacent channel interference, occurs when the nearby transmitter splashes sidebands (modulated signals) onto the frequency to which the receiver is tuned. The result of both bleedover and splatter is the same--strong bursts of noise so intense that no setting of the squelch control will eliminate them.
Control of squelch or muting of a receiver has been accomplished by means of tones in certain cases for some time. Certain FM broadcast stations transmit an inaudible tone simultaneously with certain program material; special tone sensing equipment in the receiver detects the tone and performs a squelch function by removing commercial messages from the receiver output. Other transmitters, either AM or FM, have certain messages preceded by tone transmissions consisting usually of two or more simultaneous or sequential tones. Special circuits in the receiver detect the proper tone combination and gate the receiver output so that the subsequent message can be received. The muting of the receiver can be either automatically reset after a certain time duration or manually reset.
Both of these tone actuated squelch or muting systems require some sort of special signal or tone to be transmitted by the transmitter in addition to the normal messsages or data. The squelch concept that is the subject of this application, however, does not require any special signals or tones of any kind from the transmitter originating the signals.
Toyoshima, in U.S. Pat. No. 3,939,425, discloses a noise squelching circuit that uses a phase-locked loop which is tapped to gate the receiver output.
Farrow, in U.S. Pat. No. 3,619,785, Richardson in U.S. Pat. No. 3,100,871, Stover in U.S. Pat. No. 3,358,234 and Broderick in U.S. Pat. No. 3,541,449 disclose devices broadly similar to that of Toyoshima.
McKenna in U.S. Pat. No. 2,853,601, discloses an automatic gain control circuit in which a tone injected into a modulator is subsequently detected for gain control.
A feature of the squelch circuit that is the subject of this application is that it automatically adjusts to varying noise levels.
Another feature is that in the absence of strong noise it will operate on signals so weak as to be barely detectable.
Another feature is that it will reject most splatter and bleedover interference without manual readjustment of the squelch control and bleedover will not diminish the circuit's effectiveness.
Another feature is that a preset time delay is incorporated into the squelch system which prevents erroneous squelching. Moreover, an easily detectable tone signal is used to abort the squelching operation.
This squelch circuit uses a phase-locked loop to extract a continuous tone which gates the output of the AM receiver. In one embodiment, the tone frequency modulates the intermediate frequency (I.F.) signal at the second mixer. A phase-locked loop samples the I.F. signal before the second detector and is tuned within the I.F. bandwidth. The phase comparator in the phase-locked loop generates a correction signal after comparing the intermediate frequency signal and the voltage controlled oscillator output. This correction signal contains the continuous tone signal which is detected and used to gate the audio output of the receiver.
Alternatively, the continuous tone may frequency modulate the output of the voltage controlled oscillator in the phase-locked loop. In the phase comparator this frequency modulated signal is compared to the intermediate frequency signal. If the phase-locked loop is able to track the I.F. signal, the phase comparator output will contain, as a part of the necessary correction signals, a tone frequency component which will be detected and used to gate the audio output. Since this tone is inverted in phase compared to that supplied by the tone oscillator the tone component from the phase comparator will cancel the tone from the oscillator when the phase-locked loop is operating in the locked state.
Conventional circuits may be used to detect the tone signal and gate the receiver output. A bandpass filter followed by an amplifier and detector may be used. Alternatively the bandpass filter output is amplified and fed to one input of an exclusive NOR gate. The other input is supplied by the tone oscillator. The output of the NOR gate is then inverted and filtered.